Circuit board with at least one rigid and at least one flexible area and process for producing rigid-flexible circuit boards

ABSTRACT

A rigid-flexible circuit board with two rigid areas and one flexible area, with a rigid individual layer which is copper-clad on one side, with an adhesive medium and with a copper foil, the adhesive medium having recesses in the flexible area. The rigid-flexible circuit board can be produced especially easily and economically in that at least in the rigid area there is no flexible individual layer, especially no polyimide film, between the adhesive medium and the copper foil.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a circuit board with at least one rigid areaand at least one flexible area, with a rigid individual layer which isprovided with printed conductors or is copper-clad on one or both sides,with an adhesive medium and with at least one copper foil, the adhesivemedium having recesses in the flexible area. In addition, the inventionrelates to a process for producing such a rigid-flexible circuit board.

2. Description of Related Art

Printed electrical circuits have been used for decades, for example, inelectrical devices and in motor vehicles for electronic control. Usuallythe circuit boards are rigid, and on the one hand, connect discretecomponents and highly integrated modules electrically to one another,and on the other hand, act as carriers of them. Circuit boards generallyconsist of one or more individual layers of glass fiber-reinforced,cured epoxy resin boards which are copper-clad on one or both sides forthe formation of printed circuits or conductive patterns. For multilayercircuit boards, the individual planes or the printed circuits located onthe individual layers are electrically connected to one another byplated holes in the circuit board.

In addition to rigid circuit boards, for about 30 years printed circuitshave also been used which have rigid and flexible areas next to oneanother, so-called rigid-flexible circuit boards. By providing flexibleareas, a large number of rigid circuit boards can be mechanically andelectrically connected to one another in almost any desiredthree-dimensional arrangement without plug connectors or wiring.Moreover, the flexible areas make it possible to “fold” several rigidcircuit board areas on top of one another such that a large circuitboard area and thus also a host of discrete components located on thecircuit boards can be accommodated in a relatively small space. Theflexible areas normally consist of thin polyimide films which arelikewise copper-clad on one side or both sides.

These rigid-flexible circuit boards are usually built from rigid andflexible individual layers which lie on top of one another, which extendover the entire circuit, and which are cemented to one another using anadhesive medium and pressed (EP 0 408 773 B1). There are inflexible (forexample, glass fiber-reinforced epoxy resin) and flexible insulationcarriers (for example, polyimide film) with copper cladding on one orboth sides into which the printed conductors are etched. The shape ofthe rigid layers fixes the rigid part of the circuit boards. Theflexible areas of the circuit boards are thus produced by removing apiece of the rigid layers in these areas in several process steps.

Both a liquid and a free-flowing cement which is applied for example byscreen printing and also a film with adhesive properties can be used asthe adhesive medium. In practice, in addition to pure adhesive films,especially so-called prepregs are used. A prepreg consists of aresin-impregnated glass fiber fabric, the resin not being completelypolymerized. Under pressure and heat the resin liquefies and as itsubsequently cures, it causes cementing to the bordering individuallayers. In addition to these “normal” prepregs, there are also so-calledno-flow prepregs in which flowability is reduced. In addition, so-calledcomposite films can also be used as the adhesive medium; they consistsof a flexible plastic film which is provided with adhesive on bothsides. Within the framework of this invention, both cements and also theabove described films—adhesive films, prepregs, no-flow prepregs andcomposite films—are to be included in the concept of adhesive medium.

EP 0 408 773 B1 discloses a process for producing rigid-flexible circuitboards in which before lamination of the individual layers into theoverall circuit a piece is punched out of the rigid individual layer andis arranged according to the size and the desired position of theflexible area. This piece of the rigid individual layer is then insertedagain into the individual layer to fit as closely as possible, thecomposite film in the area of the circuit board which is desired to beflexible having a recess so that the piece of the individual layer whichhad been punched out beforehand and inserted again is not cemented tothe composite film in the following process steps. After lamination, thepiece must be hermetically covered during the continuing productionprocess in order to prevent plating on the flexible area and thetransition to the rigid area. This covering is complicated andexpensive, and in spite of great care damage can occur which leads toirreparable faults so that the affected circuit board can no longer beused.

Another process for producing rigid-flexible circuit boards is knownfrom German Patent Application DE-AS 26 57 212. In this process, beforepressing the individual layers, a groove (pre-groove) is made in therigid outer layers on the side facing the flexible inside layer alongthe separating line of the rigid and flexible area of the circuit board,the groove depth being selected such that the outside of the rigidlayers remains undamaged. The composite film using which the individuallayers are cemented is cut out over the flexible area of the circuit sothat cementing of the rigid outer layer over the future flexible areadoes not take place. In addition, separating films are often insertedwhich prevent flow of the composite films during pressing. Aftercementing the individual layers and after forming the conductivepatterns on the outer layers, to produce the flexible area anothergroove (main groove) is then milled from the outer side of the rigidlayer along the separating line of the rigid and flexible area of thecircuit, this groove and the pre-groove which had already been madebeforehand being aligned to one another so that the milled-out piece canbe removed from the rigid individual layer.

Regardless of the manner in which the piece of the rigid individuallayer is removed from the area of the circuit board which is desired tobe flexible, the rigid-flexible circuit board known from EP 0 408 773 B1which underlies the invention has the disadvantage that a relativelyexpensive flexible individual layer (polyimide film) is used over theentire surface and is needed actually only for the comparatively smallflexible area. This leads to up to 75% or more of the expensivepolyimide film being “wasted”, depending on the shape of the circuitboard.

Generally, rigid-flexible circuit boards which have an area of a few 10cm² to a few 100 cm² are not produced separately, but a plurality ofsuch circuit boards are located next to one another in a rigid platewhich is generally called a panel. This panel typically has a square orrectangular surface with side lengths of for example 40 to 80 cm.Typically 20 to 30 completed circuit boards can be punched out of such apanel. The advantage of using these panels compared to using individualcircuit boards in production is that the panels can be clamped moreeasily on a work table and several circuit boards can be produced at thesame time with one clamping.

Since in these panels the flexible individual layer generally likewisehas the dimensions of the panel, in this way the proportion of “wasted”expensive polyimide film is even increased since the polyimide film isalso present in the intermediate areas between the individual circuitboards. These intermediate pieces are thrown away after the circuitboards are punched out of the panels.

For the aforementioned reasons it has been repeatedly suggested that therelatively expensive polyimide film which however has proven effectivein practice be replaced by other, more economical flexible individuallayers. German Patent DE 41 03 375 C1 discloses a rigid-flexible circuitboard in which instead of a separate composite film and a separatepolyimide film only one layer of a special flow material prepreg or apaper prepreg be used. This flow material prepreg or paper prepreg isplaced directly on the entire rigid individual layer on the circuit sideover the entire surface. Then likewise a copper foil is applied to thespecial prepreg over its entire area. To prevent the piece of the rigidindividual layer which is to be removed from sticking to the specialprepreg, a separating layer is applied to the side of the piece facingthe prepreg. A similar rigid-flexible circuit board is known from GermanPatent DE 42 06 746 C1 in which, likewise, instead of an expensiveflexible individual layer, a more economical thin prepreg is used whichcan also be bent after curing. In this circuit board, in the area whichis desired to be flexible, an insulating film is also placed between therigid individual layer and the prepreg before pressing.

German Patent Application DE 41 31 935 A1 describes a rigid-flexiblecircuit board which consists solely of an inherently rigid circuit boardmaterial so that expensive polyimide film has been completely abandoned.In this known circuit board which, however, withstands only a limitednumber of bending stresses, the rigid circuit board material in the areawhich is desired to be flexible has a much smaller thickness. Thiscircuit board which can be produced relatively easily and economicallyis limited in its range of application, on the one hand, by the onlylimited flexibility, on the other hand, it is not suited for morecomplex circuits with several planes of printed conductors.

Finally, German Patent DE 40 03 345 C1 discloses a rigid-flexiblecircuit board which has a flexible polyimide film essentially only inthe area which is desired to be flexible. Here, the polyimide film onthe side facing the copper foil is provided with a flexible adhesivelayer and on the side facing the rigid individual layer is provided withtwo thin adhesive strips which are located on the edge. The width andthe location of the thin adhesive strips must be chosen such that thepiece of the rigid individual layer which is to be removed is preventedfrom sticking to the adhesive strips. Since the thin adhesive stripsoften have a width of only 1 mm or less, the prefabrication of thepolyimide film with the adhesive strips on one side and the adhesivelayer on the other side is relatively time-consuming. Moreover, it isalso necessary for the polyimide film which has been prepared in thisway to be positioned very exactly on the rigid individual layer, sinceotherwise the piece of the rigid individual layer which is to be removedcan likewise stick.

SUMMARY OF THE INVENTION

The object of this invention is thus to make available the initiallydescribed rigid-flexible circuit board and to devise a process forproducing such a rigid-flexible circuit board, in which, on the onehand, the use of expensive flexible individual layers is for the mostpart abandoned, but which, on the other hand, can be produced as easily,quickly and economically as possible.

The aforementioned object, first of all, is achieved essentially in theinitially described circuit board in that at least in the rigid areathere is no flexible individual layer between the adhesive medium andthe copper foil. Since, generally, most of the circuit board is formedby the rigid area, the measure in accordance with the invention resultsin that a large part of the expensive flexible individual layer can beomitted. If there is no polyimide film in the rigid area of the circuitboard, this moreover has the advantage that special treatment, forexample, by means of plasma cleaning, of the holes in the circuit boardwhich are plated to connect the individual planes of a multilayercircuit board, can be abandoned.

Advantageously, in the circuit board of the invention, a flexibleindividual layer, especially a polyimide film, as the carrier for thecopper film can be completely omitted. Polyimide films have thedisadvantage that they absorb moisture so that when circuit boards arebeing produced and assembled several time-consuming drying processes areoften necessary to prevent damage to the circuit board by delaminationof the cemented areas or peeling of the printed conductors in a laterworking step. If polyimide film is completely eliminated, the dryingprocesses which are otherwise necessary can be completely omitted.

According to one preferred embodiment of the circuit board in accordancewith the invention, in the flexible area on the inner side of the copperfoil, an insulating layer is applied directly to the copper foil.Basically, a host of materials can be used for the insulating layer. Theinsulating layer, on the one hand, must ensure insulation of the copperfoil to the outside and of the individual printed conductors among oneanother, on the other hand, it must have sufficient flexibility so thatthe flexibility of the flexible area of the circuit board is notadversely affected to a significant degree by the insulating film.Finally, the insulating layer must have a composition such that itadheres directly to the copper foil. The insulating layer can beimplemented especially easily by a flexible resist, especially a solderresist. These resists are very economical, thus absorb essentially nomoisture, so that the drying processes can be omitted, and moreover theycan be applied very quickly and easily at the desired location to thecopper foil using a host of known processes.

According to a last advantageous configuration of the circuit board ofthe invention which will only be briefly addressed here, at least in theflexible area, another insulating layer is applied to the outer side ofthe copper foil. Likewise, the insulating layer can also be applied tothe entire outer surface of the copper layer. In doing so, a flexiblesolder resist can in turn also be used as the insulating layer, forexample. This second insulating layer on the outer side of the copperfoil protects the conductive pattern which has been produced in thecopper foil by etching and in particular mechanically stabilizes it inthe flexible area. Another known cover film can also be used hereinstead of the solder resist.

The circuit board according to the invention is suited not only forproducing two-layer, rigid-flexible circuit boards, but also forproducing so-called multilayer circuit boards which have a host oflayers with at least one conductive pattern in each layer. Therefore,the invention also relates to a multilayer circuit board with at leastone rigid area and at least one flexible area, consisting of at leastone above described circuit board as claimed of the invention, therebeing several rigid individual layers which are provided with printedconductors or which are copper-clad on one or both sides, and/or severalcopper foils which are provided with an insulating layer in the flexiblearea, and the rigid individual layers being cemented among one anotherand/or the copper foils being cemented among one another and/or therigid individual layers and the copper foils being cemented to oneanother by means of an adhesive medium.

In the initially described process for producing rigid-flexible circuitboards, the aforementioned object is achieved according to a firstalternative in that first an uncured insulating layer is applied to oneside of the copper foil in the flexible area, that the insulating layeris cured to such an extent that the free surface loses its adhesivecapacity, that afterwards the copper foil which has been pretreated inthis way is cemented by means of the adhesive medium to the rigidindividual layer, the side of the copper foil with the insulating layerfacing the rigid individual layer, and that, then, in the flexible areaof the circuit board, a piece of the rigid individual layer is removed.Because the insulating layer is partially cured after application to thecopper foil, by which the free surface of the insulating layer loses itsadhesive capacity, it is ensured that the piece of the rigid individuallayer which is to be removed does not adhere to the insulating layer, bywhich the subsequent removal of the piece would be made difficult orentirely prevented.

According to one alternative configuration of the process in accordancewith the invention, the copper foil is first cemented to the rigidindividual layer by means of an adhesive medium, then in the flexiblearea of the circuit board, a piece of the rigid individual layer isremoved and then an uncured insulating layer is applied to the innerside of the copper foil in the flexible area.

In the two above described alternative processes, the finished circuitboard in the flexible area has simply a copper foil which is covered onthe inside by an insulating layer. In the first alternative of theprocess of the invention, first, this insulating layer is applied to thecopper foil before cementing and pressing of the circuit board, while inan alternative embodiment of the process according to the invention,first, the circuit board is cemented and pressed, then in the flexiblearea a piece of the rigid circuit board is removed and only then is thenow accessible area of the copper foil provided with an insulatinglayer.

According to one advantageous configuration of the process of theinvention the insulating layer which can be, for example, a flexiblesolder resist, is sprayed, rolled or printed onto the copper foil. Allthese techniques for applying the insulating layer to the copper foilcan be carried out easily and economically, and by using thecorresponding templates the insulating layer can be applied to thecopper foil exactly at a defined position. In addition, with thesemethods, at the same time, a plurality of insulating layers can beapplied to a large-area copper foil, so that in, one process step, thecopper foil can be prepared for a complete panel, i.e., for a host ofcircuit boards.

In particular, there are a host of possibilities for embodying anddeveloping the circuit board in accordance with the invention and theprocess of the invention. In this regard reference is made to thefollowing description of a preferred embodiment in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic section of a rigid-flexible circuit board knownfrom the prior art and

FIG. 2 shows a schematic section of one embodiment of a rigid-flexiblecircuit board in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a section through a known circuit board 1 (FIG. 1)and through a circuit board 1 in accordance with the invention (FIG. 2).The circuit board 1, which is shown only schematically in the figuresand not yet cemented and pressed, will have in the fully assembled statetwo rigid areas 2 and one flexible area 3 which connects the two rigidareas 2 to one another. To achieve this flexible area 3, after pressingthe circuit board 1, a piece 5 is milled out of the rigid individuallayer 4. To do this, two pre-grooves 6 are formed in the rigidindividual layer 4 on the inner side.

In addition to the rigid individual layer 4 which is copper-clad on oneside in the embodiment shown, both the known circuit board 1 and alsothe circuit board 1 in accordance with the invention have an adhesivemedium 7 and a copper foil 8. The adhesive medium 7 which is generallyformed from a prepreg, in the flexible area 3 has a recess so that thepiece 5 does not become cemented to the adhesive medium 7.

In the known circuit board 1 as shown in FIG. 1, moreover, there isanother flexible individual layer 9 which is used as the carrier for thecopper foil 8 and which is cemented to the rigid individual layer 4 bymeans of the adhesive medium 7 and is pressed. This flexible individuallayer 9 is made mostly of a relatively expensive polyimide film which,on the one hand, has proven effective as a carrier of the copper foil 8in millions of rigid-flexible circuit boards, but which, on the otherhand, has other disadvantages in addition to the price which is highcompared to the rigid individual layer 4. Due to the fact that thepolyimide film absorbs moisture, both in the manufacture of the circuitboard 1 and also during later assembly, several drying processes areoften necessary to prevent damage to the circuit board 1, especially tothe conductive patterns, by the evaporation of the absorbed liquid.Moreover, the use of a polyimide film as a flexible individual layer 9in the rigid area 2 of the circuit board 1 also causes increased cost inthe cleaning of the holes 10 made in the circuit board 1. The holes 10are used to connect the individual conductive patterns in the differentplanes of the circuit board 1 to one another. For this reason, the holes10 are plated, for which, however, special cleaning of the holes 10 isnecessary beforehand and requires, for the most part, special treatmentin the form of plasma cleaning when the polyimide film is drilledthrough.

In the circuit board of the invention which is shown in FIG. 2,conversely, a flexible individual layer 9, especially a polyimide film,has been completely eliminated. The copper foil 8 is attached directlyto the rigid individual layer 4 by means of the adhesive medium 7. Toprevent unallowable electrical contacts of the copper foil 8 with othercomponents or short circuits between the individual printed conductorswhich have been formed on the copper foil 8, a thin insulating layer 11is applied to the inner side, i.e., on the side facing the rigidindividual layer 4, of the copper foil 8. To ensure sufficientinsulation, the insulating layer 11 projects slightly over the surfaceof the piece 5 of the rigid individual layer 4 which is to be milledout.

In one preferred embodiment of the circuit board 1 according to theinvention and of the process of the invention, for producing arigid-flexible circuit board 1, the insulating layer 11 is applied tothe copper foil 8 before the circuit board 1 is cemented and pressed. Todo this, for example, a flexible plastic resist, especially a flexiblesolder resist, can be sprayed or imprinted onto the corresponding siteon the copper foil 8. Using methods such as spraying, printing orrolling, the insulating layer 11 can be applied to the desired locationsof a copper foil 8 very easily, promptly and accurately in terms ofposition. In doing so, at the same time, a host of insulating layers 11can be applied to a copper foil 8 of a corresponding size so that in oneprocess step a copper foil 8 can be prepared for several circuit boards1.

According to one alternative—however not shown here—process forproducing a rigid-flexible circuit board 1 in accordance with theinvention, it is also possible to apply the insulating layer 11 onlyafter cementing and pressing the circuit board 1 onto the inner side ofthe copper foil 8. In this process, before applying the insulating layer11, first the piece 5 is removed from the rigid individual layer 4, sothat the inner side of the copper foil 8 in the flexible area 3 of thecircuit board 1 becomes accessible. In addition, it is alsofundamentally possible to completely eliminate the use of an insulatinglayer 11 if the danger of unallowable electrical contacts or shortcircuits is precluded in some other way, for example, by potting acircuit board 1 which has been inserted into a housing.

The circuit board 1 according to the invention, moreover, has anotherinsulating layer 12 which, after cementing and pressing of the copperfoil 8 to the rigid individual layer 4, is applied to the outer side ofthe already structured copper foil 8. In the circuit board 1 of theinvention as shown in FIG. 2, this insulating layer 12—in contrast tothe known circuit board 1 as shown in FIG. 1—does not have a polyimidefilm as a partial cover film 13 either. The insulating layer 12 can inturn be formed from a flexible solder resist which is sprayed onto thecopper foil 8 either only partially in the flexible area 3 of thecircuit board 1 or over the entire surface, except for the holes 10.

The circuit board 1 in accordance with the invention, which is shown inFIG. 2, is thus characterized by especially simple, fast and economicalproduction. First of all, money is saved by the relatively expensivepolyimide film being completely eliminated. Moreover, application of theinsulating layers 11 and 12 which are composed of, for example, ofsolder resist in a spraying or printing process is possible very easily,precisely and quickly.

As a result of the different coefficients of expansion of the rigidindividual layer 4 and the polyimide film which is used in the priorart, in the circuit boards 1 which are used in the prior art, whentemperature fluctuations occur, undesirable sagging of the circuit board1 and a varied expansion in the thickness of the circuit board 1 betweenthe rigid areas 2 and the flexible area 3 often occurs; this can lead tocracks in the printed conductors or in the plated holes 10. In thecircuit board of the invention, conversely, the material of theinsulating layer 11, 12 can be chosen such that the circuit board 1 iscomprised essentially of materials with similar coefficients ofexpansion. As another advantage, this yields more uniform expansionbehavior of the circuit board 1 so that the circuit board 1, under theaction of heat, has only minimum allowable warping and bulging. In thisway, the circuit board 1 as in accordance with the invention can be usedespecially effectively, for example, in automobiles, in which majortemperature fluctuations often occur.

1. Circuit board, comprising: a rigid individual layer which is providedwith printed conductors or is copper-clad on at least one side, saidrigid individual layer having at least one rigid area and at least oneflexible area, at least one copper foil, an adhesive medium directlyconnecting the at least one copper foil to the rigid individual layer,and wherein there is no flexible individual layer between the adhesivemedium and the at least one copper foil in the rigid area of the rigidindividual layer, and an insulating layer is affixed directly to aninner side of the copper foil in the at least one flexible area of therigid individual layer, wherein the adhesive medium has an opening inalignment with the flexible area of the rigid individual layer, wherein,in the flexible area of the rigid individual layer, there is no flexibleindividual layer between the rigid individual layer and the copper foil,and wherein the insulating layer is a resist applied to the copper foil.2. Circuit board as claimed in claim 1, wherein the resist is a flexiblesolder resist.
 3. Circuit board as claimed in claim 1, wherein, at leastin the flexible area, another insulating layer is applied to the outerside of the copper foil.
 4. Circuit board as claimed in claim 3, theother insulating layer is a resist which has been applied to the copperfoil.